1. Field of the Invention
The present invention relates to exposure control systems for photographic apparatus, in general, and to the stepper motor control of a dynamic aperture or scanning type shutter mechanism in a photographic camera, in particular.
2. Description of the Prior Art
A scanning type shutter mechanism operated under the control of a photoelectric circuit that measures and integrates ambient scene light level, and provides a trigger signal at a predetermined percentage of optimum exposure value to initiate the shutter mechanism closing phase of a film exposure interval is well known in the prior art.
Typically, a scanning type shutter mechanism includes a pair of shutter blade elements that are displaceable between a light blocking first arrangement where the exposure aperture is closed and a second arrangement where the shutter blade elements cooperate to define a maximum available exposure aperture. As the blade elements open, the area of the exposure aperture progressively increases until a peak aperture is reached and then the direction of each blade element displacement is reversed causing the exposure aperture area to progressively decrease until it closes to terminate the exposure interval. The peak aperture is the largest aperture opening that is achieved during the course of an exposure interval and it may be smaller than the maximum available aperture. In an ambient or natural available light mode of operation the peak aperture is generally correlated to ambient scene light level. In an artificial or flash mode of operation, peak aperture is typically selected in accordance with both camera-to-subject distance and ambient scene light level inputs.
During the ambient mode of operation a light detection and integration circuit monitors the scene light level as the blade elements open and provides a signal at a predetermined percentage of optimum exposure value to trigger the blade closing phase. Because of inertial characteristics of the blade mechanism, and its associated drive system it is impossible to achieve instant blade mechanism closure. Thus, there will be some finite time between the generation of a trigger signal to command blade mechanism closure and movement of the blade mechanism to its fully closed position, during which time the exposure will continue. If the blade mechanism has not reached the maximum exposure aperture defining position where the blades are at rest when the trigger signal is provided, it will also take some time for blade deceleration and reversal of the drive direction. In other words, there will be some overshoot time which also must be accounted for to achieve total control over the exposure process.
Several scanning type shutter mechanism drive systems incorporating means for anticipating the degree of the shutter blade mechanism overshoot during an exposure interval and compensating for same have been disclosed in the art. In one type of scanning type shutter mechanism such as that described in U.S. Pat. No 3,942,183 to Whiteside, the shutter mechanism is driven between its open and closed positions by a drive system incorporating an electrical solenoid and a spring arrangement. The shutter mechanism includes a pair of overlapping shutter blade elements interposed between an objective lens and a film plane. The solenoid and spring arrangement is coupled to each of the shutter blade elements through a pivotable link or walking beam. Scene light admitting primary apertures are provided in each of the shutter blade elements. Each of these primary apertures are selectively shaped and positioned such that they overlap the central optical axis of the taking lens to collectively define a gradually varying primary opening when actuated between their open and closed positions by the above-mentioned solenoid and spring drive system.
Each of the blade elements in the shutter mechanism also include at least one secondary aperture. The secondary aperture in one blade cooperates with the secondary aperture in the other blade to form a secondary opening therethrough. These cooperating secondary apertures are configured to track in a corresponding relationship with respect to scene light admitted through the primary apertures. With the primary and secondary apertures being formed in the same blade element and therefore being mechanically coupled to one another the secondary apertures move in the same manner as the primary apertures when the blade elements of the blade mechanism are displaced by the above-mentioned solenoid and spring drive, with respect to one another.
The amount of light admitted to the film plane through the primary apertures is controlled by an integrated signal generated by a photosensor that senses scene light through the secondary apertures. Compensation for the amount of primary aperture size overshoot that would otherwise occur during an exposure interval is provided, in this particular shutter mechanism drive system, by increasing the size of the secondary apertures over what they would have been absent the above-described blade mechanism overshoot problem. By increasing the size of the secondary apertures in this manner more scene light will be sensed by the photosensor through these apertures during an exposure interval. An end of exposure signal is generated by the integrator coupled to the output of the photosensor relatively early in the exposure interval to thereby anticipate and compensate for excessive exposure causing blade mechanism overshoot.
In another type of scanning type shutter mechanism such as those described in U.S. Pat. Nos. 4,325,614, 4,354,748 and 4,427,276, the shutter mechanism is displaceably driven by a stepper motor. The shutter mechanism includes a pair of overlapping shutter blade elements interposed between an objective lens and a film plane as in the above-noted Whiteside patent. Stepper motor control is provided by an automatic exposure control system that includes a microcomputer preprogrammed with data representing a plurality of different blade trajectory signal programs for different photographic conditions. The exposure control system includes a blade position encoder for sensing relative blade element position throughout an exposure interval. The blade position information provided by the blade position encoder is utilized, in part, to compensate for any potential excessive exposure that would otherwise be caused by blade mechanism overshoot. It should be noted that U.S. Pat. No. 4,325,614 to Grimes, cited above, also discloses an open loop exposure control system wherein the stepper motor is controlled by a plurality of different blade trajectory signal programs previously stored within a microcomputer. However, this particular embodiment makes no provision for the excess exposure problem created by blade mechanism overshoot.
Each of the overshoot compensating scanning type shutter mechanism drive systems mentioned above has one or more disadvantages. The solenoid and spring driven shutter mechanism is relatively bulky and is therefore unsuitable for use in a small or compact photographic camera. The stepper motor driven shutter mechanism is compact and is therefore quite suitable for use in a small or compact camera. However, the blade position encoder system, including that portion of the exposure control system associated therewith, increases exposure control system cost and complexity.